In known systems, an inverter amplifier is coupled to a resonating element, for example a quartz crystal or a ceramic resonator, and a pair of load capacitors, which must be charged before the system will start to oscillate. The inverter amplifier is generally provided on an integrated circuit, for example in CMOS technology, and the resonating element is a discrete component provided off chip. The capacitors may be on or off chip. With relatively high power, the capacitors are charged to operating levels relatively quickly, so that the startup time is fairly low. However, if the operating power needs to be reduced to low levels, for example in portable, battery-operated devices, such as watches, pagers or cellular telephones, then the charging time of the capacitors is increased, in some cases to unacceptable levels.
An analysis of the effects contributing to the total start-up time of conventional push-pull inverter oscillators is known from Rusznyak, A.; "Start-Up Time of CMOS Oscillators", IEEE Trans. on Circuits and Systems, March 1987. In this known analysis it is shown that the time to charge crystal network capacitances depends partly on the value of feedback bias resistance. Most known circuits use a high value feedback resistance for d.c. bias, thus severely limiting the speed with which oscillations can start-up.
Oscillator circuits which incorporate control circuits to ensure oscillator start-up and regulate oscillation amplitude or supply current are known. One particular approach is described in Vittoz, E. A.; Quartz Oscillators for Watches, Proc. Int. Congress Chronometry, 1979. A similar approach is described in Vittoz, E. A. et al.; High-Performance Crystal Oscillator Circuits: Theory and Application, IEEE J. Solid-State Circuits, June 1988. Both these approaches are based on current source inverter amplifiers. The principle of these known circuits is to generate a negative feedback signal which is a measure of a.c. oscillation amplitude, and use it to control amplifier current, and hence also amplifier transconductance and resulting oscillation amplitude. Another particular approach is disclosed in Swiss Patent CH 640 698, awarded to Luscher, J. on Jul. 3, 1984, and is based on appropriate control of complementary current sources. The principle of this known circuit is to dynamically control variable output current at the minimum level required to drive sinusoidal circuit oscillations. All the above approaches aim to ensure that oscillations do start, and that they are subsequently sustained at minimum power drain, for all conditions of crystal resistance, MOS fabrication process, and operating environment, thus also providing maximum frequency stability. However, it is not the aim of these known circuits to reduce oscillation start-up time.
It is therefore an object of the present invention to provide an amplifier circuit for connection to a resonating element to provide clock signals, which overcomes, or at least reduces, the disadvantages of the prior art.